A conventional screw pump typically includes an elongated pump cover having a fluid inlet located adjacent a first longitudinal end, or “suction side,” thereof, and a fluid outlet located adjacent a second longitudinal end, or “discharge side,” thereof. A rotatably driven screw, commonly referred to as a “power rotor,” and two or more intermeshing, non-driven “idler rotors” extend through the pump cover and operate to entrain and drive fluid from the fluid inlet to the fluid outlet. An end of the power rotor on the discharge side terminates in a balance piston that separates the discharge side of the pump from a cavity at low pressure further downstream, typically serving as seal chamber and being connected with the suction side of the pump. In some configurations, the balance piston may abut and limit axial movement of the idler rotors. The power rotor extends through a ball bearing that supports the power rotor and allows the power rotor to rotate freely about its axis with minimal frictional resistance. Alternatively, a slide bearing may be implemented which also may incorporate the function of the balance piston.
During operation, the idler rotors of a screw pump may be subjected to significant hydraulic and frictional forces that require axial counter-balancing to hold the idler rotors in place. Various mechanical arrangements have been implemented for providing such counter-balancing. For example, in screw pumps having a “hanging idler” configuration, which is particularly suitable for handling low pressures and/or high viscosity fluids, the balance piston of the power rotor is radially flanked by low pressure chambers defined by downstream ends of idler rotor bores formed in the pump cover. These low pressure chambers are located immediately downstream from the downstream faces of the idler rotors and thus allow pumped fluid to flow downstream beyond the idler rotors with relatively little resistance. The back pressure at the downstream faces of the idler rotors is therefore relatively low, resulting in a relatively small net axial force on the idler rotors directed toward the discharge side. Since the net axial force is relatively small, axial engagement between the downstream faces of the idler rotors and the upstream face of the balance piston may be sufficient to counter-balance the axial force and stabilize the idler rotors. Additionally, other forces (e.g., gravity) that may act on the idler rotors during assembly and/or reorientation of the pump are relatively small in this configuration and may be counteracted by simple counter-balancing faces integrated into the pump cover to restrict axial movement of the idler rotors toward the suction side.
Thus, the hanging idler configuration is relatively inexpensive and can be readily implemented in a modular, easily removable rotor assembly, though such configuration is generally not suitable for handling high pressures and/or low viscosity fluids for which the leakage over the balance piston, acceptable in the hanging idler configuration and resulting in lower volumetric efficiency, may not be acceptable, and for which greater counter-balancing may be necessary.
For applications in which it is necessary to handle high pressures and/or low viscosity fluids, and/or if it is desirable to mitigate leakage of a pumped fluid, a screw pump having a “thrust face” configuration may be implemented. In contrast to the hanging idler configuration described above, the thrust face configuration employs an arrangement in which the entire circumference of the balance piston is surrounded by the pump cover in a radially close-clearance relationship (i.e., with no low pressure chambers flanking the balance piston as in the hanging idler configuration), thereby substantially preventing fluid leakage around the balance piston. This arrangement creates significant backpressure at the discharge side, resulting in a relatively large net axial force on the idler rotors directed toward the suction side. Since axial engagement between bearing surfaces of the power rotor and the idler rotors and/or between bearing surfaces of the pump cover and the idler rotors may not be sufficient to counter-balance the net axial force and stabilize the idler rotors, alternative counter-balancing structures at the upstream ends of the idler rotors on the suction side may be necessary. For example, the suction side of the pump cover may be provided with bearing surfaces, or “thrust faces,” against which the upstream ends of the idler rotors may bear during operation. Thus, while the thrust face configuration provides reduced leakage relative to the hanging idler configuration, it does so at the expense of greater frictional losses resulting from engagement between the idler rotors and the thrust faces of the pump cover. Additionally, the structural elements necessary for implementing the thrust face configuration increase the cost and complexity of the configuration. Still further, if the thrust faces are incorporated into the pump cover, the thrust face configuration generally cannot be implemented in a modular, easily removable rotor assembly.
For applications in which it is necessary to handle high pressures and low viscosity fluids having poor lubrication properties, a screw pump having a “balance bushing” configuration may be implemented. The balance bushing configuration employs an arrangement in which an end of each idler rotor (typically the end on the suction side) is tapped and is surrounded by a bushing. Fluid lines that are internal or external to the pump cover are used to channel an amount of the pumped fluid from an opposing end of the idler rotors to the tapped ends via holes in the bushings, whereby the channeled fluid provides a counter-balancing, axial force on the idler rotors. Since the pressure of the pumped, low viscosity fluid is subject to dramatic variation, it is generally necessary to employ additional counter-balancing structures (e.g., thrust disc arrangements) on the opposite ends of the idler rotors (i.e., the ends of the idler rotors opposite the ends on which the balance bushings are disposed). These additional counter-balancing structures, along with the fluid lines that are necessary for channeling the pumped fluid to the balance bushings, make the balance bushing configuration the most complex and most expensive of the above described screw pump configurations. Additionally, if the balance bushings are disposed on the suction side of the screw pump, a modular, easily removable rotor assembly generally cannot be implemented.
In view of the foregoing, it would be advantageous to provide a modular, easily removable rotor assembly for screw pumps, wherein the rotor assembly is capable of handling high pressures and low viscosity fluids without requiring the costly and complex counter-balancing structures of conventional thrust face and balance bushing screw pump configurations.